1. Field of the Invention
The present invention relates to a method for forming a type II cellulose. More specifically, it relates to a method for forming a type II cellulose employing water in a supercritical or subcritical state.
2. Description of the Related Art
Heretofore, it has been known that use of water in a supercritical or subcritical state can induce a variety of unique reactions. Japanese Patent No. 3042076 discloses the hydrolysis of cellulose using water in a supercritical or water in a subcritical state as a solvent, whereby the cellulose can be decomposed into an oligomer or glucose.
A cellulose molecule has a structure as shown in FIG. 1 and is a homopolysaccharide formed by a β-(1,4) glycosidic linkage of β-D-glucose residues. In general, cellulose molecules form a strong crystal state by intramolecular hydrogen bonds or intermolecular hydrogen bonds with other cellulose molecules.
The main crystal structures of celluloses are a type I cellulose and a type II cellulose, which are shown in FIG. 2 and FIG. 3, respectively. In FIGS. 2 and 3, the drawing on the left side shows an arrangement within a fiber plane, and the drawing on the right side shows an arrangement between fibers. As can be understood from these drawings, both the crystal structures of the type I and type II celluloses have a monoclinic system, and the arrangements of their molecules within a fiber plane ((2,0,0) plane) are about the same. However, the molecules of the type I cellulose between the fiber planes form a parallel arrangement, while the molecules of the type II cellulose between the fiber planes form an antiparallel arrangement. Therefore, they are different from each other.
Most celluloses existing in nature have the type I cellulose structure, and those having the type II cellulose structure exist on rarely in nature. The type II cellulose is produced by causing the cellulose to swell in a mixture of various organic solvents and then protonating the swollen cellulose, and thus the produced type II cellulose is partially utilized as synthetic fiber.
As described above, the type II cellulose is scarce in nature and is only produced by a specific process, and the thus produced type II cellulose is restrictively used. However, in actual fact, a variety of type II celluloses having a different degree of polymerization exist, and it is considered that the type II celluloses would be employed in a wider variety of fields if they could be easily produced. Particularly, conventional production methods comprise decomposition of the type I cellulose by use of a mixed organic solvent containing an acid or the like and has a problem that other chemical substances are mixed in and a procedure to eventually obtain the type II cellulose thereby becomes complicated.